I have a warm air heating system with a gas fed exchanger for the hot water. For a while now i have had loud banging noises from the system. It would appear to be a circulation problem as the cold water feed gets hot and switches of the water heat before it reaches temprature.I have improved the problem by draining down the system and refilling from fresh, but after a couple of months it starts again.I have had a lot of gravel type scale from the system, could the hot water tank be blocked?. It is 30 yrs old.